A particular form of ophthalmic surgery known as radial keratotomy requires making incisions in the cornea of the eye. These incisions must be made accurately and to no more than a selected depth. Surgical knives designed for this purpose often include a type of micrometer drive or mechanism to enable the surgeon to extend the cutting surface of the blade beyond a footplate or guide formed on the knife by a distance equal to the selected depth of the cut.
Due to the delicate nature of ophthalmic surgery, it is desirable to provide highly accurate depths of cut and to provide backups or double checks of the depth of cut. This is particularly so when the depth of cut may be set prior to surgery. When the depth of cut must be changed during surgery, it is advantageous to provide a means for making the change quickly, conveniently and in a manner which allows the depth of cut to be verified. It is also an advantage to have a system for verifying or setting depth of cut that makes it easier to maintain the sterility of the instruments being used.
Prior efforts of others to provide a means for setting the depth of cut of such knives include apparatus such as block gauges or coin gauges, wherein the knife is positioned against a gauge and the blade is then extended until it physically contacts the gauge. Examples of such prior art gauges are found in U.S. Pat. No. 4,662,075 (Mastel, et al.), at FIGS. 1A and 1B.
Use of mechanical gauges with micrometer knives has certain inherent disadvantages. The blades on such knives (typically formed from diamond or sapphire) must be extremely sharp to avoid tearing the tissue of the eye during surgery. This means using a diamond or sapphire knife blade which, while extremely sharp, are also susceptible to damage if the tip of the blade is abutted against a hard surface such as that of the gauge. Mastel, et al. also point out that the use of such gauges also creates the risk of parallax errors in reading the gauge scales, and that error also results over time as the gauges suffer material degradation or wear.
Instead of such mechanical depth-setting gauges, Mastel, et al. teach and describe a microscope with a stage having a holder for the knife and a field of magnification within which the tip of the knife blade is positioned and viewed. According to the teachings of Mastel, et al., the microscope's field of view has a reticle axis which appears to the user as a vertically-extending line. This axis is used as a line of reference for setting the depth of cut.
The knife to be set is clamped into a holder mounted to a tray which, in turn, is mounted to the stage of the microscope in such a manner as to allow the tray to be moved in relation to the stage by a screw-and-spindle mechanism operable through use of a micrometer dial in a direction perpendicular to that of the reticle.
The knife preferably includes a micrometer handle, the operation of which allows the knife blade to be extended from or withdrawn into the handle. When placed in the holder, the knife's footplate or guide faces the reticle, and the knife blade is fully retracted. The micrometer handle is then used to operate the spindle and advance the knife toward the reticle until the footplate or guide is aligned with the reticle. At this point, the micrometer which controls the motion of the tray is "zeroed" that is set to a zero scale reading and is then rotated to move the tray away from the reticle by a distance equal to that of the depth of the cut. Next, the micrometer handle of the knife is rotated to advance the knife blade while the user observes the reticle through the microscope eyepiece. When the blade tip just reaches the reticle, it is extended beyond the footplate or guide by a distance equal to the depth of the cut and is then removed from the holder to be used in surgery. In this manner, the depth of cut may be set without physical contact between the blade and a mechanical guide and without the concomitant risk of damage to the blade tip.
Use of the Mastel, et al. device calls for a number of physical operations. The knife to be used is first secured to a tray mounted on a moveable stage which, in turn, is mounted to a base. The stage can be moved in both the x- and y-axis direction with respect to the base. A micrometer spindle is used to move the stage in the x-axis direction.
A microscope is mounted to the same base and is positioned to allow the movement of the table to bring the knife blade into the microscope's field of vision. Typically, the knife is of the type having a micrometer mechanism used to adjust movement of the knife blade into or out of the knife handle, and a footplate beyond which the blade must be extended for cutting. Once the knife blade is within the microscope's viewing field, the table is moved to align the knife's footplate with a reticle formed on the microscope lens. This requires that the surgeon concentrate on focusing the microscope on the footplate and, thereafter, manipulating the micrometer to advance the knife blade to align the blade tip with the reticle as well. The surgeon then "zeroes" the micrometer spindle on the moveable stage and then uses the spindle to move the stage along an axis perpendicular to the reticle for a distance equal to the selected depth of cut. Thereafter, the surgeon uses the knife's micrometer mechanism to advance the blade until the blade tip again aligns with the reticle. With the cutting depth now set, the knife is removed from the tray and is ready to use. A brief description of the Mastel, et al. device is found in the '075 patent at Column 3, lines 3-41.
Use of the foregoing method and apparatus requires the surgeon to peer into the microscope during initial setting and any eventual verification of the depth of cut setting, at a time when the surgeon is gowned, masked and gloved, and is striving to keep a sterile operating field. The surgeon risks contamination of the gloves he or she is wearing every time a micrometer spindle is manually touched and adjusted. Mastel, et al. do make provisions for using sterilized sleeves to cover the microscope's operating controls, but this creates yet another potential source of contamination and another set of details to consider at a time when the surgeon is already preoccupied with the details and procedures of surgery.
Even if the knife, or knives, are adjusted prior to surgery, the surgeon may well wish to verify the depth of cut immediately prior to making an actual incision. While Mastel, et al., describe several ways to verify the setting obtained with the microscope, they involve prior art procedures such as viewing the micrometer setting on the knife or using a "certified" gauge block mounted to the table.
U.S. Pat. No. 4,750,489 (Berkman, et al.), teaches and describes a radial keratotomy (RK) knife of the general type described above and which has a linear variable differential transformer incorporated as part of the knife structure. The knife is inserted into an electrical console which is used to set the depth of cut by measuring the position of the transformer within the knife. A method for zeroing the blade utilizes a membrane inside the console is contacted by the knife's footplate and a laser to project a beam focused at the footplate. An optical detector detects changes in the beam's characteristics which occur when the knife blade is advanced to contact the membrane and thus signals the "zero" point of the knife.
U.S. Pat. No. 4,744,362 (Grundler) teaches and describes a device for performing keratoplasty automatically rather than by hand and which features a monitor used by the machine operator to observe the various steps of the operation as the machine carries them out.
U.S. Pat. No. 4,922,909 (Little, et al.) teaches and describes a video monitoring system for use in surgery where a computer is used to store tissue images which can then be retrieved and compared to the appearance of the tissue at later stages of treatment or surgery.
U.S. Pat. No. 4,691,715 (Tanne) teaches and describes an automatic corneal surgery system using a series of probes to map the surface of the cornea. A computer then processes the information and uses it, along with information as to the electrical resistivity of the cornea, to control the movement and depth of cut of a cutting blade. Depth of cut is set by the vertical movement of the blade.
U.S. Pat. No. 5,098,426 teaches and describes a method and apparatus for precision laser ocular surgery utilizing a surgical video microscope which projects an image to a video monitor. A series of ruling lines projected onto the tissue surface provides further information as to tissue dimensions, contours, etc. sufficient to guide a laser beam used as part of surgery.
There are also known high resolution video measuring systems such as the VIA.RTM.-100 Video Measurement System sold by Boeckler Instruments, Inc. of Tucson, Ariz., and the Imagen HR 1024.TM. sold by Optech Instrument Corporation of Greenvale, N.Y. Both are "edge-finding" optical systems using a video microscope to produce a high-resolution video image fed to a processing unit which, when calibrated, can accurately measure the object being scanned.
Against this background of prior art, there exists the need for a calibration system useful to set the cutting depth on micrometer-adjustable surgical knives to be used manually during surgery.
The need also exists for such a system which can be conveniently used by a surgeon during surgical procedures to set, reset or verify the cutting depth of a particular knife without unnecessarily interrupting surgery or risking violation of the sterile field.
The need further exists for such a system to be adaptable for use with a wide variety of surgical knives.
The need also exists for such a system to zero and set the cutting depth on surgical knives without physical contact between the system and the knife tip.
The need also exists for such a system to allow for verification or change of the cutting depth while minimizing the efforts needed to maintain sterility of the instrument and the system components.